As heating equipment grew in size and complexity, the potential for dangerous operating conditions resulting in property loss and even loss of life, increased proportionally.Municipalities, States, and the Federal Government became involved in passing laws and codes to ensure installed heating equipment would meet minimum safety standards.Insurance companies that covered losses which resulted from fires or explosions of defective or improperly installed burners, formed organizations of “underwriters”.Underwriters assess the risk of coverage of an insurance policy, and in order to minimize losses, developed standards of safety.In the commercial/industrial field of burners, these various underwriters developed codes for burners rated 400,000 BTU/HR input or more.

Some of the various underwriters are:

Underwriters Laboratories (U/L)

Factory Mutual (FM)

Industrial Risk Insurers (IRI)

National Fire Protection Association (NFPA)

Improved Risk Mutual

Kemper

And although not an underwriter, the American Society of Mechanical Engineers (ASME) has developed a national code, ASME CSP-1.

UL, FM, and IRI (formerly FIA) are the major underwriters.

A building’s owner(s) will purchase insurance from an insurance company who will have an underwriter share the loss risk.Concerning burners, this underwriter will periodically inspect the premises to ensure the burners in the building meet their standards of safety.If not, they will recommend what needs to be done to upgrade the equipment.Usually, the building owner has a choice.He can ignore the recommendations, in which case his premiums will increase because the loss risk is greater, or he can do the recommended upgrade, which will keep his premiums down, maybe even lower them, often enough to pay for the upgrade in a few years.

IRI has the strictest standards for fuel trains.If a fuel train is designed to IRI specifications, you can be assured it meets FM and UL standards.

Figure A is a typical gas fuel train.(The pilot system is not shown.)

Figure A.

Figures I to III show the components to meet UL and FM underwriter requirements according to the BTU/HR input.Figures IV to VI are IRI’s standards.Figure VII summarizes the differences between UL, FM, and IRI.(The proper flame safeguard controls need to be applied to the various systems to complete an installation.)

Figures I through VII

Proper operation of a natural gas-fired forced draft burner is dependant on a properly selected and assembled gas train.The gas train requirements are dictated by such factors as:

1)The approval agency (UL, FM, IRI, etc.)

2)Gas pressure available, flowing, at the inlet connection

3)Burner input

4)Required burner manifold gas pressure

Figure 8.

Figure VIII shows the location of components in a typical gas train, a “block and bleed” system.(The two SSOV’s are the “blocking” valves; the N.O. vent valve, the “bleed” valve.)Figure VIII also shows what constitutes the total DP and DP from the outlet of the pressure regulator of a gas train.To a certain extent, the designer of the gas train will determine DP.DP is important.By using a high DP, component parts will be smaller and consequently lower priced than components sized by using a low DP.But there are operating problems that must be addressed.If a burner has a high turn down ratio, a low DP, below 2" W.C. must be used to maintain good control.An on/off burner with no turn down can use a high DP.Because of the DP variable, sizing a gas train is as much an art as it is a science.

After gathering the four requirements previously mentioned, the gas train designer will pick a size he feels will work well, both from an operation and cost standpoint, and then check the DP using the components selected.If the DP is too great for proper system operation, he will re-size the gas train, and check the DP again.Generally, on/off systems (no firing rate valves) can be designed using a higher DP than systems with firing rate valves.On systems with firing rate valves the designer will pick a DP based on the gas inlet pressure.

Gas Inlet PressureSuggested DP

3" WC to 6" WC.5" WC - 1.5" WC

7" WC to 14" WC1" WC - 2" WC

14" WC to 1 psi2" WC - 5" WC

2 psi and up6" WC - 10" WC

Pressure drops indicated are based on wide-open valves.If the valve is less than fully open, higher drops will result.

For competitive reasons, burner manufacturers that supply gas trains may use higher DP’s than shown in the chart.Higher DP’s than recommended may result in noisy operation and poor performance.

Manufacturers print graphs, charts, or tables showing pressure drops at various flow rates of individual components used in assembling gas trains.Tables are also available showing pressure drops for piping.

Some manufacturers do not print graphs showing DP’s at various flow rates, but only at one or two specific flow rates.Often a graph will be difficult to read at certain specific flow rates.See Figure IX.Which line represents the 3000 CFH?Which line is 10,000 CFH?Obviously, the 10,000 CFH line is easiest to identify.The designer can then use 10,000 CFH as a basis to find the DP’s for all the gas train components and then by applying a simple formula, find the actual DP for the gas train at the actual flow rate wanted.

Figure XI.

As an example:

Size an IRI gas train for a 3,000 MBH input burner, where 1 Lb. gas pressure is available, the burner manufacturer recommended manifold pressure is 4" WC, and there is a firing rate valve.Figure IV illustrates the components needed for this gas train.

Using the following components and DP’s at 10,000 CFH:

ComponentDP at 10,000 CFH

BB2-100-2" Shutoff valve1.41

RV91-2" Regulator6.27

V5055-2" SS0V6.00

V5055-2" SS0V6.00

BB2-100-2" Shutoff valve1.41

Q Type Butterfly Valve1.68

10 Ft. Schd. 40 Blk. Iron Pipe3.96

Total DP @ 10,000 CFH26.73"

Divide the required input MBH by the standard flow rate used, then square the result

Multiply the result by the total DP originally found at 10,000 CFH:

26.73 x .09 = 2.41 WC.

2.41" WC is the DP through the gas train selected for the 3,000 MBH burner.

The selected 2" components for this burner’s gas train will work very well.There is low enough DP so our modulating firing rate valve will work well, plenty of pressure is still available for the required 4" WC manifold pressure, and no excessive noise will be created.Had the DP exceeded 5" WC, a recalculation using 2 1/2" components would be done.

Climatic Control Company, Inc. has engineers in I.S.D. who are experienced in fuel train design.You can be confident that our engineers have the ability to design an excellent performing, cost effective, fuel train.